U.S. patent number 3,978,294 [Application Number 05/587,060] was granted by the patent office on 1976-08-31 for return answer-supervisory circuit.
This patent grant is currently assigned to Bell Telephone Laboratories, Incorporated, Western Electric Company. Invention is credited to Albert David Limiero, Edwin Walter Lipien.
United States Patent |
3,978,294 |
Limiero , et al. |
August 31, 1976 |
Return answer-supervisory circuit
Abstract
A PBX system is disclosed having a return answer supervisory
circuit in a local PBX for automatically detecting and extending an
answer indication from a called station located in a distant PBX to
an attendant's console in the local PBX on tie trunk calls between
the local and distant PBXs. The disclosed system is an improvement
in the art in that it does not require major modification to
existing equipment for returning the answer indication to the
attendant's console.
Inventors: |
Limiero; Albert David
(Longmont, CO), Lipien; Edwin Walter (Boulder, CO) |
Assignee: |
Western Electric Company (New
York, NY)
Bell Telephone Laboratories, Incorporated (Murray Hill,
NJ)
|
Family
ID: |
24348176 |
Appl.
No.: |
05/587,060 |
Filed: |
June 16, 1975 |
Current U.S.
Class: |
379/225;
379/240 |
Current CPC
Class: |
H04Q
3/625 (20130101) |
Current International
Class: |
H04Q
3/62 (20060101); H04M 007/06 () |
Field of
Search: |
;179/27CA |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
War Department Technical Manual TM 11-335, Telephone Central Office
Set TC-1, 15 Apr., 1942 pp. 39-43. .
Proc. of 1970 International Conf. on Communications: "Solid State
Miniature Tie Trunk Circuit" by R. R. Maxon pp. 43-47 thru 43- 16,
1970..
|
Primary Examiner: Cooper; William C.
Attorney, Agent or Firm: Duft; D. M.
Claims
What is claimed is:
1. In a telephone switching system having a local PBX, a distant
PBX, tie trunks interconnecting said local PBX with said distant
PBX, an attendant's console having a plurality of lamps at said
local PBX, a matrix type switching network in said local PBX, tie
trunk circuitry connected to one side of said network and attendant
console control circuitry connected to another side of said
network, means for extending a call over one of said tie trunks
from said local PBX to a called station in said distant PBX, said
local PBX further having a return answer supervisory circuit
comprising:
means responsive to said extension for signaling one of said lamps
on said attendant's console in a first unique code,
means responsive to the answering of said call by said called
station for transmitting an attendant lamp control signal from said
tie trunk circuit through said network to said attendant console
control circuitry, and
means responsible to said transmitted signal for signaling said
lamp in a second unique code.
2. The system of claim 1:
in which said means for signaling said lamp in a first unique code
comprises a relay having make contacts and break contacts and a
power source for delivering power according to said first unique
code through said break contacts to said lamp; and
in which said means for signaling said lamp in a second unique code
comprises a power source for delivering power according to said
second unique code through said make contacts to said lamp when
said relay is operated, said relay being operated when said called
station in said distant PBX answers.
3. The system of claim 1 in which said tie trunk comprises tip and
ring leads and said answer condition is a reversal of battery
polarity on said tip and ring leads when said called station
answers.
4. The system of claim 3:
in which said means for signaling said lamp in a first unique code
comprises a relay having make contacts and break contacts and an
intermittent power source for delivering intermittent power through
said break contacts to said lamp when said relay is released;
and
in which said means for signaling said lamp in a second unique code
comprises a direct current power source for delivering direct
current power through said make contacts when said relay is
operated in response to said battery polarity reversal
condition.
5. In a telephone switching system having a local PBX, a distant
PBX, tie trunks interconnecting said local PBX with said distant
PBX, said local PBX having an attendant's console with a plurality
of lamps, central office trunk circuits, a switching network, and
an interface circuit, said distant PBX having a plurality of
stations, means in said local PBX for detecting the receipt of a
call appearing on one of said central office trunk circuits and
requiring access to a called one of said stations in said distant
PBX, means including said attendant's console for establishing a
path including a sleeve lead path from said one central office
trunk circuit through said network to said interface circuit, and
said local PBX further having a return answer supervisory circuit
comprising:
means in said one central office trunk circuit responsive to the
establishment of said sleeve lead path for signaling one of said
lamps in said attendant's console in a first unique code,
means in said interface circuit interconnected with the called
station in said distant PBX over said tie trunk circuit for
detecting when said called station answers, and
means responsive to said answer detection for signaling said lamp
in a second unique code.
6. The system of claim 5 in which said means for signaling
comprises:
a relay having make contacts and break contacts,
means responsive to said answer detection for operating said
relay,
a first power source for delivering power according to said first
unique code through said break contacts to said lamp when said
relay is released, and
a second power source for delivering power according to said second
unique code through said make contacts to said lamp when said relay
is operated.
7. A switching system comprising:
a local PBX,
a distant PBX having a plurality of stations,
a tie trunk interconnecting said PBXs,
a tie trunk circuit connected to said tie trunk at said local
PBX,
an attendant console at said local PBX,
a central office connected to said local PBX via a central office
trunk circuit,
a switching network in said local PBX,
means in said local PBX for detecting the receipt by said central
office trunk circuit of a call from said central office requiring a
connection to a called station at said distant PBX, means including
said attendant console for establishing a network path between said
central office trunk circuit and said tie trunk circuit,
means effective upon said path establishment for extending said
call over said tie trunk to said distant PBX,
means in said local PBX responsive to the establishment of said
network path for displaying a first signal at said attendant
console,
means in said local PBX for detecting an off hook state of the
called station at distant PBX, and
means responsive to said detection for displaying a second signal
at said attendant console.
8. In a PBX having central office trunk circuits, interface tie
trunk circuits, a switching network, common control equipment, and
an attendant's console having a plurality of lamps, means including
said common control equipment and said attendant's console
effective upon the receipt of an incoming call by one of said
central office trunk circuits for establishing a network connection
from said one central office trunk circuit to an idle one of said
interface tie trunk circuits, said one interface tie trunk circuit
being interconnected with a called station located in a distant PBX
in response to said network connection, means in said one central
office trunk circuit for signaling one of said lamps in a first
unique code upon the establishment of said network connection, and
means in said interface tie trunk circuit responsive to an answer
signal from said called station for maintaining the tie trunk
connection to said called station, and improvement comprising:
first means responsive to said network connection for inhibiting
the provision of a second unique code signal by said signaling
means to said lamp,
second means responsive to said network connection for signaling
said lamp in a first unique code, and
means interconnected with said first means responsive to said tie
trunk maintenance for enabling said signaling means to signal said
lamp in said second unique code and for inhibiting said second
means.
9. The system of claim 8:
wherein said signaling means for signaling said lamp in said second
unique code comprises a relay having make contacts and a second
power source for delivering power according to said second unique
code through said make contacts to said lamp when said relay is
operated; and
wherein said inhibiting means comprises means responsive to said
network connection for shunting said relay so that said relay
remains released.
10. The system of claim 9 wherein said second means comprises:
break contacts in said relay, and
a second power source for delivering power according to said second
unique code through said break contacts to said lamp when said
relay is released.
11. The system of claim 10 wherein said enabling means
comprises:
a current source, and
means responsive to said tie trunk maintenance for connecting said
current source to said relay so that said relay operates.
12. A method for providing return answer supervision in a PBX
having central office trunk circuits, a network, interface tie
trunk circuits, common control equipment, and an attendant's
console with a plurality of lamps, said common control equipment
being controlled by said attendant's console upon the receipt of a
call on one of said central office trunk circuits for establishing
a connection in said network from said one central office trunk
circuit to an idle one of said interface tie trunk circuits, said
one interface tie trunk being interconnected with a called station
located in a distant PBX in response to said network connection,
means in said one central office trunk circuit for signaling one of
said lamps in a first unique code upon the establishment of said
network connection, and means in said interface tie trunk circuit
responsive to an answer signal from said called station for
maintaining the tie trunk connection to said called station, said
method comprising the steps of:
inhibiting the provision of a second unique code signal by said
signaling means to said one lamp in response to said network
connection,
signaling said one lamp according to a first unique code in
response to said network connection,
inhibiting the provision of the first unique code signal to said
one lamp in response to said tie trunk maintenance, an
enabling the provision of the second unique code signal by said
signaling means to said one lamp in response to the inhibiting of
said first signal.
13. The method of operating a PBX having central office trunk
circuits, a switching network, interface tie trunk circuits, common
control equipment, and an attendant's console with a plurality of
lamps, said method comprising the steps of:
detecting the receipt by one of said central office trunk circuits
of a call that requires a connection to a called station in a
distant PBX,
establishing under control of said attendant's console a network
connection from said one central office trunk circuit to an idle
one of said interface tie trunk circuits,
shunting the coil of a relay in response to said network connection
so that said relay is prevented from operating, said relay being
provided for signaling one of said lamps through make contacts of
said relay in a second unique code upon the answering of said call
at said distant PBX,
signaling said one lamp according to a first unique code through
break contacts of said relay in response to said network
connection,
supplying current to the coil of said relay in response to an
answer signal from said distant PBX on said one interface tie trunk
circuit so that said relay operates, and
signaling said one lamp according to said second code when said
relay operates.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a telephone system and, in particular, to
a private branch exchange (PBX) system having equipment for
automatically providing return answer supervision on tie trunk
calls between two PBXs.
2. Description of the Prior Art
It is common practice to provide supervisory circuits of various
types and configurations in PBX systems in order to implement
useful and valuable customer services and features. One such
particular feature is desired when a call to a local PBX requests a
station located in a distant PBX. An attendant located at the local
PBX receives the call on an incoming central office trunk and then
transfers the call over a tie trunk to the distant PBX.
Prior art arrangements provide a return supervisory indication to
the attendant when the common control of the local PBX establishes
a network connection between the calling central office trunk and
an idle tie trunk. This return supervisory indication activates a
previously unlit lamp on the attendant's console to indicate that
the network connection is made and that the calling line is
transferred to an idle tie trunk. If the lamp remained unlit, the
attendant could further service the calling party in the event, for
example, the tie trunks are all busy.
Although prior art arrangements operate satisfactorily to provide
supervision to the point of the network connection, the call is
denied attendant supervision on tie trunk calls after the tie trunk
is seized. For example, when the called party does not answer, in
prior art arrangements, the calling party must place a second call
to the local PBX attendant if the calling party desires further
service.
BRIEF SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide improved
supervisory return indications on tie trunk calls in a private
branch exchange system.
It is a further object to provide a return answer supervisory
circuit that is adaptable to prior art arrangements with a minimum
of cost and complexity.
SUMMARY DESCRIPTION
Prior art supervisory arrangements for tie trunk calls between
local and distant PBXs are less than ideal since they provide the
attendant with only an indication of whether or not the tie trunk
is connected by the network to the calling central office trunk of
the local PBX. Thus, calling stations incoming to the local PBX
over central office trunks are denied attendant supervision after
the tie trunks are connected. This includes the time required for
the calls to be processed by the distant PBX.
We provide a return answer supervisory circuit for central office
trunk calls which require a minimum of modification to an existing
prior art arrangement in order to provide the valuable feature of
attendant supervision for the calling station until the called
station answers.
Our invention is disclosed as embodied in a local PBX having tie
trunks over which calls may be transferred to a distant PBX. An
incoming call over a central office trunk to the local PBX desiring
a station located in the distant PBX is directed to the attendant's
console. The attendant answers the call, determines the directory
number of the called station, and dials the called station's number
into the common control of the local PBX. The common control
selects an idle tie trunk, establishes a network connection between
the calling central office trunk and the tie trunk in the local
PBX, and outpulses the called station's number to the distant PBX.
The distant PBX receives the called station's number over the tie
trunk and provides ringing current to the called station.
In accordance with our invention, a return answer supervisory
circuit delivers a flashing signal indication to a lamp located in
the attendant's console commencing with the completion of the
network connection between the tie trunk and the central office
trunk and lasting until the called station answers at which time
the signal indication changes to a steady illumination. In the
event that the called station does not answer, the attendant,
noticing that the lamp is still flashing, can effect a connection
into the calling central office trunk and determine if the calling
party requires further service.
FEATURES
One feature of the present invention is to provide return answer
supervision in a local PBX for signaling an attendant when a called
party located in a distant PBX answers a call directed over a tie
trunk from the local PBX.
Another feature of the present invention is to provide return
answer supervision for tie trunk calls between a local PBX and a
distant PBX by providing a first signal to the attendant when the
call is extended by the local PBX to the distant PBX, and a second
signal to the attendant when the called party at the distant PBX
answers.
A further feature of the present invention is the provision of a
shunt path across a relay used for detecting the network connection
of a tie trunk call thereby preventing the relay from operating a
signal lamp on the attendant's console and the provision of an
additional current source which is applied to the relay when the
called party in a distant PBX answers the tie trunk call causing
the relay to operate the signal lamp.
DRAWING
These and other objects and features of the invention will become
more apparent upon a reading of the following description thereof
taken in conjunction with the drawing in which:
FIG. 1 discloses a block diagram of a private branch exchange
system and associated apparatus for returning answer supervision
configured in accordance with one illustrated embodiment of our
invention;
FIG. 2 illustrates a prior art arrangement of returning supervision
in the local PBX upon the completion of the network connection for
a tie trunk call and a prior art arrangement for detecting an
answer signal by the called station; and
FIG. 3 discloses modifications, in accordance with this one
illustrative embodiment of our invention, to the prior art
arrangements of FIG. 2 to provide return answer supervision for a
tie trunk call.
GENERAL DESCRIPTION
FIG. 1 is a block diagram depicting a telephone system having a
central office 14, a local PBX 20 with an attendant's console 40,
and a distant PBX 50.
In an effort to simplify the description as much as possible
consistent with the full disclosure of our invention, only two
telephone stations 10 and 60 are illustrated. Moreover, only a
single central office trunk 16, a single tie trunk 54, and a single
attendant's console 40 are shown. However, in an actual telephone
switching system, a plurality of such trunks, consoles and distant
PBXs would be utilized to service a plurality of such telephone
stations. For example, a typical telephone system of the present
invention may comprise thirty to forty central office trunks and
one to three attendant's consoles in the local PBX, five to ten tie
trunks to a distant PBX, and one to three distant PBXs.
The operation of the depicted telephone switching system in FIG. 1
is described only to the extent that a full understanding of our
invention can be realized. For a more comprehensive understanding
of tie trunk calls between local and distant PBXs, reference can be
made to "Solid State Miniature Tie Trunk Circuit," (sic) R. R.
Maxon, Proceedings, 1970 International Conference on
Communications, pg. 43-7 to 43-20.
As is well known in the telephone art, a calling station 10 over
tip and ring leads 12 can gain access through central office 14 and
over a central office trunk 16 to a local PBX 20. If the calling
station 10 desires connection to a called station 60 at a distant
PBX, the common control 28 of the local PBX 20 effects an
interconnection between the tip and ring leads of the central
office trunk 16 with the tip and ring leads of an idle tie trunk 54
in a conventional manner which will be more fully discussed in the
following. In a manner well-known in telephony art, the distant PBX
50 interconnects the calling station 10 now appearing on the tip
and ring leads of the tie trunk 54 with the tip and ring leads 56
of the called station 60.
The interconnection in the local PBX 20 of a conventional central
office trunk 16, with the tie trunk 54 occurs in the following
manner. The central office trunk circuit 18 interconnects the tip
and ring leads of the central office trunk 16 with the tip and ring
leads of leads 38. As will be more fully discussed, the sleeve lead
S of leads 38 provides a convenient electrical path for a variety
of supervisory signals including those of the present
invention.
The tip, ring and sleeve leads 38 appear on the switching network
22 which may comprise a conventional crossbar switch as disclosed,
for example, in "Basic Telephone Switching Systems," Pages 104-107,
by David Talley (Hayden, 1969). The network 22 establishes, under
attendant control, a path from the tip, ring and sleeve leads 38 to
the tip, ring and sleeve leads 35 which input into a conventional
tie trunk interface circuit 24.
The interface circuit 24 interconnects the ring, tip and sleeve
leads 35 with the tip, ring and sleeve leads 39 for delivery into a
conventional tie trunk circuit 26. The tie trunk circuit 26
connects the tip and ring leads of leads 39 with the tip and ring
leads of the tie trunk 54.
The common control 28, in a well known fashion over control leads
30, 32 and 34, directs the establishment of tip and ring paths, T
and R, from the central office trunk 16 to the tie trunk 54 and
further directs the establishment of a sleeve lead path S from the
central office trunk circuit 18 to the tie trunk circuit 26 as
shown with dotted lines in FIG. 1. The above circuits and
interconnections are conventional and concern our invention only in
a manner to be subsequently described.
It is important to note that prior to the establishment of the tip,
ring and sleeve paths in FIG. 1, the calling station 10 was
interconnected with the attendant's console 40 over path 19,
extending between the central office trunk circuit and the
attendant's console 40. During the establishment of this path, the
calling party on station 10 is in voice communication with an
attendant at the attendant's console 40 during which time the
identity of the called station 60 is determined by the attendant.
The attendant, by operating the attendant's console 40 in a manner
not shown in detail, signals the common control 28 over lead 42 to
establish the above-described path from the central office trunk 16
to the tie trunk 54. Details of this approach are well known and
they do not comprise the essence of this invention.
The attendant's console 40 provides a real time interactive control
by an attendant for servicing a plurality of such tie trunk calls.
For the purposes of the present invention, only one signal lamp 48
is shown in FIG. 1. Signal lamp 48 may derive its illumination
power from either of two sources located in the central office
trunk circuit 18: a direct current source 44 activated by closing
make contacts SL-2 and a 60 indications per minute (IPM) source 46
activated through break contacts SL-1. Either power source, 44 and
46, can be delivered over lead 52 to the lamp 48 and then to
ground.
In a manner to be subsequently described in detail, the prior art
arrangement which provides only tie trunk supervision operates in
the following manner. Before the establishment of the tip, ring and
sleeve paths, as shown by dotted lines in the local PBX 20, the
lamp 48 is not illuminated from any power source. When common
control 28 selects an idle tie trunk circuit 26 in a conventional
fashion, a relay SL, not shown in FIG. 1, is operated which closes
the make contacts SL-2 thereby delivering direct current power over
lead 52 to the lamp 48 from source 44 to provide the attendant with
a steady illumination. This indication to the attendant conveyed
the information that a tie trunk to central office trunk connection
has been made by the network 22 and that the call has been routed
over the tie trunk 54 to the called station 60. The attendant at
this time would provide no further supervision for the tie trunk
call. The prior art approach did not utilize the 60 IPM power
source 46 for supervision at this time. The 60 IPM power source 46
is used, however, in prior art arrangements to indicate CAMP-ON
(i.e., when a third party desires access to a first party engaged
in a conversation with a second party).
In accordance with our invention and in a manner to be detailed
later, we provide a 60 IPM signal to lamp 48 from source 46 by
adding break contacts SL-1. This signal continues during the
selection of an idle tie trunk 54, it further continues during the
establishment of the tip and ring path from the central office
trunk 16 to the tie trunk 54, and it further continues during the
establishment of a path over the tie trunk 54, through the distant
PBX 50 and to the called station 60. This 60 IPM signal for lamp 48
terminates only when the called party 60 goes off hook (i.e.,
answers) at which time a reverse battery signal on the tip and ring
leads is delivered from the called station 60, through the distant
PBX 50 and over the tie trunk 54 into the tie trunk interface
circuit 24. Upon detecting the reverse battery signal in the
interface circuit 24, a relay SL, not shown in FIG. 1, is operated
to switch the power source for lamp 48 from source 46 to source 44
in order to provide a steady illumination for the attendant.
Therefore, according to our invention, we have enabled the
attendant to continue supervision of a tie-trunk call until the
called party answers with only minor modification of existing
circuitry.
It is therefore apparent, that our invention enables an attendant
located at the attendant's console of the local PBX 20 to maintain
supervision for a tie-trunk call until the called party answers.
This feature provides a valuable service for calling station 10,
since if the called station 60, the distant PBX 50, the tie trunk
54, or the tie trunk interface circuit 24, would fail or be busy,
the attendant would notice a "no answer" condition and would be
able to reconnect to the calling party to provide additional
service.
DETAILED DESCRIPTION
FIG. 2 shows a prior art arrangement in which network supervision
is returned to the signal lamp 48 of the attendant's console 40. As
previously discussed for FIG. 1, the lamp 48 is not illuminated
until an idle tie trunk 54 is seized for a calling central office
trunk 16 by the common control 28 in a conventional manner.
Concurrent with the network connection of the tie trunk 54 to the
central office trunk circuit 18, a relay SL in trunk circuit 18 is
operated to connect the direct current power source 44 to the lamp
48 thereby providing the attendant a steady illumination. This
steady illumination informs the attendant that a tie trunk is
seized.
FIG. 2 illustrates only one example of prior art circuitry
necessary for network supervision on tie trunk calls. The tip and
ring interconnections are only shown diagramatically since their
details are well known to those skilled in the art. These leads
provide a conventional voice transmission path for the calling
party. The prior art arrangement of FIG. 2 and our invention of
FIG. 3 are both concerned only with the circuitry of the sleeve
lead S which interconnects the tie trunk interface circuit 24, the
switching network 22, and the two-way trunk circuit 18.
In the prior art arrangement of FIG. 2, two separate circuits are
illustrated. The first circuit is shown in heavy bold lines and it
involves an interconnection between the central office trunk
circuit 18 and the switching network 22 on sleeve lead S. The
second circuit involves an interconnection between the tie trunk
interface circuit 24 and the switching network 22, and it is shown
by normal width lines for sleeve lead S.
The first circuit comprises a prior art arrangement for providing
network supervision for a tie trunk call. After the common control
28 of FIG. 1 selects an idle tie trunk 54 for a calling central
office trunk circuit 18, the next processing step establishes a
network path from trunk circuit 18 to the tie trunk circuit 26.
When that network path is made for the tip and ring leads, T and R,
a corresponding network connection is made for the sleeve lead S,
as shown in FIG. 2.
The network connection is conventional and occurs when a ground,
not shown, is applied to node 64 by the common control 28. A ground
appearance at node 64 causes current to flow through the hold
magnet coil HM thereby operating the hold magnet HM. Crosspoints
HM-1 and HM-2 are thereby closed causing the hold magnet HM to be
held by current from the ground in the central office trunk circuit
18, through closed contacts SA-1 and relay SL on sleeve lead S.
Relay SA, shown symbolically, operates to perform unrelated duties
whenever a call is being served by the central office trunk circuit
18. Therefore, at the time of the network connection, make contacts
SA-1 are closed and hold magnet HM is thereby held operated. When
the call is ended, relay SA releases to release hold magnet HM.
Relay SL operates upon the removal of the ground at node 64 and
causes make contacts SL-2 to close thereby connecting the direct
current power source 44 to lamp 48. This connection, as described
above, provides a steady illumination to signal lamp 48 thereby
informing the attendant that a network connection has been made.
Prior to the network connection, lamp 48 was not illuminated.
Relay TT in the central office trunk circuit 18 operates upon the
seizure of an idle tie-trunk by the common control and such action
becomes important in the implementation of our invention. However,
relay TT is primarily used for other purposes in the prior art
arrangement of FIG. 2 which are completely unrelated to our
invention. Relay TT releases at the termination of the call.
The second circuit of the FIG. 2 becomes activated when the called
station 60 goes off-hook (i.e., answers). An off-hook condition in
the called station 60 reverses battery polarity on the tip and ring
leads 54 of the distant PBX 50 in a conventional fashion. This
battery reversal is delivered over the tip and ring leads 54 and
into the interface circuit 24 where it is detected by a battery
reversal indication circuit 67.
Upon a battery reversal condition on the tip and ring leads 39,
relay DS operates and closes make contacts DS-1. With contacts DS-1
closed, a conducting path for sleeve lead S is provided in the
following manner. The minus 48 volt potential in the interface
circuit 24 is delivered through the winding of relay RV, through
make contacts DS-1, over sleeve lead S of leads 35, through the
closed network crosspoint connection HM-1, through the closed
network crosspoint connection HM-2, over sleeve lead S of leads 38,
through the relay SL winding and through make contacts SA-1 to
ground. Diodes 60 provide electrical isolation for relay SL from
other signals appearing on sleeve lead S which are not shown.
The establishment of this conducting path S operates relay RV which
closes make contacts RV-1. Relay RV is now held operated by its own
contacts RV-1. This hold path is released by the opening of
contacts SA-1 when the call terminates as previously discussed.
Relay DS is now able to change state, as for example, when the
called station 60 generates a flash signal (i.e., a monentary
on-hook signal). However, despite the state of relay DS, relay RV
remains operated. With relay RV operated for the duration of the
call, make contacts RV-2 close and provide a current path through
the tip and ring leads 39 via diode 69 such that during the flash
indication and during the on-hook momentary condition the
connection to the distant PBX 50 is maintained rather than lost. It
is understood that in conventional telephony art, current must be
maintained in the tip and ring path 49 to maintain the tie trunk
connection.
Therefore, when the called station 60 provides a first off-hook
condition, a battery reversal is detected by relay DS which
operates relay RV. Relay RV remains operated for the duration of
the call and relay DS is free to detect other signaling conditions
of the called station 60 including that of a flash condition. With
relay RV operated, contacts RV-2 are closed thereby maintaining
current through the tip and ring leads for the duration of the
flash by the called station 60.
FIG. 3 illustrates one embodiment of our invention which utilizes
the existing circuitry of the prior art arrangement of FIG. 2 as
extensively as possible. Such a design goal of minimizing hardware
additions to an existing system is essential in keeping telephone
system costs at a minimum in order to provide subscribers with
inexpensive phone service while at the same time providing
additional valuable features. These design goals are met in the
present embodiment.
Additions to the existing circuitry of FIG. 2 are shown in heavy
bold lines in FIG. 3 and includes a resistor 80 in parallel with
the coil of relay RV in the tie trunk interface circuit 24 and the
inclusion in the central office trunk circuit of a shunt path 74
which comprises two diodes 71, a resistor 72, and make contacts
TT-1 of relay TT. In addition, break contacts SL-2 are provided
between the 60 IPM source 46 and the lamp 48. The system operation
of our invention will now be discussed.
When the common control 28, shown in FIG. 1, has seized an idle tie
trunk circuit 26 in order to establish a connection for the calling
station 10 appearing in the central office trunk circuit 18, relay
TT is operated indicating that a tie trunk circuit is seized. Relay
TT as mentioned in the previous discussion of the prior art, serves
other functions which are not related to the concept of our present
invention. However, when relay TT operates to indicate that a tie
trunk is seized, the closure of make contacts TT-1 places the shunt
path 74 across the coil of relay SL.
Therefore, when the network crosspoint connection HM-2 is made on
the sleeve lead S by the common control 28, a potential source of
minus 48 volts in the network 22 is delivered through the coils of
the hold magnet HM, through the crosspoint connection of HM-2, over
the sleeve lead path S of leads 38, and to a bifurcated electrical
path comprising one branch containing the relay SL and a second
branch containing the resistor 72. The summation of the current
entering and leaving node 61 must equal zero so that:
however, the current I1 through the coil of the relay SL is
insufficient to operate relay SL. With relay SL released, the
signal lamp of FIG. 1 receives its power from the 60 IPM source 46
since break contacts SL-1 remain closed. As previously mentioned
relay SA operates when the central office trunk circuit 18 serves a
call for which a network connection has been established,
therefore, contacts SA-2 are closed when the central office trunk
circuit 18 is serving a call for which a network 22 connection has
been established. At the termination of the call, contacts SA-2
release thereby preventing delivery of any power to lamp 48.
In comparison to the prior art approach of FIG. 2, our invention,
as illustrated in FIG. 3, prevents the operation of relay SL by
providing a shunt path 74 which reduces the current of the prior
art approach through coil SL to a value below the value of current
necessary to activate relay SL.
At this stage of the call, the calling station 10 has been
interconnected with a tie trunk 54 in the local PBX 20. The distant
PBX 50 will effect a connection from the tie trunk 54 to the called
station 60 and will begin ringing the called party 60. Therefore,
while the called station 60 is ringing, current I3 in the direction
indicated on sleeve lead S of leads 38 enters from the Central
office trunk circuit 18 into the minus 48 volt potential source of
network 22. It is important to note that at this time in the
progress of the call, current in the sleeve lead S of leads 35
interconnecting the network 22 with the tie trunk interface circuit
24 is nonexistent. This is due to the fact that while the network
crosspoint connection HM-1 is closed, the make contacts DS-1 and
RV-1 are open since relays DS and RV are not operated during this
stage of the call.
When the called party 60 goes off-hook (i.e., answers), battery is
reversed by the distant PBX 50 on the tip and ring leads 54 and is
delivered into the tie trunk interface circuit 24.
As previously discussed in the prior art approach of FIG. 2, the
battery reversal indication circuit 67 detects battery reversal and
operates relay DS to close make contacts DS-1. With contacts DS-1
closed, an electrical path is established beginning with a minus 48
volt potential in the tie trunk interface circuit 24 which is
delivered through the coil of relay RV, through the now closed
relay contacts DS-1, over the sleeve lead S of leads 35, through
the network crosspoint connection of HM-1, through the network
crosspoint connection of HM-2, over sleeve lead S of leads 38 and
to the bifurcated branch connection of SL and resistor 72 and
thence to ground.
The above electrical path from minus 48 volts to ground causes
current to flow in the coil of relay RV thereby operating relay RV
to close make contacts RV-1. With contacts RV-1 closed, the relay
RV is held operated to ground. In this manner, relay DS may release
without effecting the loss of current in the coil of relay RV. It
is important to note at this time, an additional resistance 80 is
in parallel with the coils of RV in order to increase the amount of
the current I5 in sleeve lead S. The value of current I5 in sleeve
lead S of leads 35 is essential to the operation of this invention
as will become more apparent in the ensuing discussion.
At this stage of the call, the called station 60 has gone off-hook
in response to ringing thereby causing the relay RV to operate and
to cause the current I5 to flow. The current I5 increases the value
of current I3 by an amount equal to I5 as shown in the current node
equation at node 64:
therefore,
this means that current I1 and current I2 proportionately increase
together by an accumulative amount of I5. The respective increase
in current I1 is sufficient to bring the current flowing through
the coil of relay SL over the value of the current required to
operate relay SL. Relay SL operates upon the appearance of current
I5 to close make contacts SL-2 and to open break contacts SL-1.
With contacts SL-2 closed, the signal lamp 48 now receives its
power from direct current source 44 and provides a steady
illumination to the attendant.
In accordance with our invention, when the called station 60 goes
off-hook, an additional current I5 is provided in the sleeve lead
path S of the local PBX 20 which increases the current flowing
through relay SL to a value which effects relay SL to operate.
Relay S1 operated conveys a signal to the attendant that the called
station 60 has answered.
The above described invention of FIG. 3 and the prior art
arrangements of FIG. 2 find application in direct termination PBXs.
That is PBXs in which, at a given time, only one of the several
attendant's consoles has direct access to a given tie trunk call.
Many modern PBXs have shared loop access wherein each attendant's
console has access through a network to each tie trunk call. The
above invention may be readily adapted by one skilled in the art to
find application in a shared loop PBX wherein the attendant serving
the call would not be dropped from the loop until the called party
answers.
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